This invention relates generally to steam turbines, and more particularly, to controlling steam leakage paths in the turbine.
A steam turbine may include a high-pressure (HP) turbine section, an intermediate-pressure (IP) turbine section, and a low-pressure (LP) turbine section that each include rotatable steam-turbine blades fixedly attached to, and radially extending from, a steam-turbine shaft that is rotatably supported by bearings. The bearings may be located longitudinally outwardly from the high and intermediate-pressure turbine sections. A steam pressure drop through at least some known high-pressure and/or intermediate-pressure turbine sections is at least about 2,000 kPa (kiloPascals), and a difference in pressure of the steam entering the high and intermediate-pressure turbine sections is at least about 600 kPa. In some known steam turbines, steam exiting the HP turbine section is reheated by a boiler before entering the IP turbine section.
A steam turbine has a defined steam path which includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. Steam leakage, either out of the steam path, or into the steam path, from an area of higher pressure to an area of lower pressure, may adversely affect an operating efficiency of the turbine. For example, steam-path leakage in the turbine between a rotating rotor shaft of the turbine and a circumferentially surrounding turbine casing, may lower the efficiency of the turbine leading to increased fuel costs. Additionally, steam-path leakage between a shell and the portion of the casing extending between adjacent turbines, for example, a high pressure turbine section to an adjacent intermediate turbine section, may lower the operating efficiency of the steam turbine and over time, may lead to increased fuel costs.
To facilitate minimizing steam-path leakage between the HP turbine section and a longitudinally-outward bearing, and/or between the IP turbine section and a longitudinally-outward bearing, at least some known steam turbines use a plurality of labyrinth seals. Such labyrinth seals include longitudinally spaced-apart rows of labyrinth seal teeth. Many rows of teeth are used to seal against the high-pressure differentials that may be in a steam turbine. Brush seals may also be used to minimize leakage through a gap defined between two components, such as leakage that is flowing from a higher pressure area to a lower pressure area. Brush seals provide a more efficient seal than labyrinth seals, however, at least some known steam turbines, which rely on a brush seal assembly between turbine sections and/or between a turbine section and a bearing, also use at least one standard labyrinth seal as a redundant backup seal for the brush seal assembly.
Other areas of steam path leakage within a turbine may affect adversely turbine efficiency. One such area is a casing fit between the HP turbine section and the IP section where labyrinth and brush seals are impractical.